I found the answer to my own question !-)

In Thunderbird, on the 'Account Settings' page, in the lower left-hand corner, find the 'Account Actions' drop-up button; click on it to expose a list of actions including 'Remove Account'.

I plan to remove my Yahoo account from Thunderbird, then close my Yahoo account, in an attempt to stop a recurring Yahoo pop-up -- which looks like it might be a web page, but actually appears in a Thunderbird window! -- even when no web browser is running! I documented this problem in Bugzilla #1680658 'Recurring Rogue Thunderbird Window Phishing for Yahoo password'.

Thanks. Plase mark your aswer as 'Solution'.